Spray, butter, or line with aluminum foil a 9x13-inch baking dish or pan.
Heat the butter on medium-low heat until just about melted.
Add the mini marshmallows to the melted butter. Cook on medium-low heat until the marshmallows are melted, stirring constantly.
Add the Cocoa Krispies. Mix thoroughly. Try not to mush up the Cocoa Krispies.
Spread the marshmallow and Cocoa Krispie mixture into the prepared baking dish.
Very lightly pat down the Cocoa Krispie mixture into the baking dish.
Let the Krispie treats cool for about an hour, then cut into squares and serve.
Notes
It is best to eat the Cocoa Krispies treats within a day or two. The longer they sit uneaten, the harder they will get.
You can freeze the Cocoa Krispie treats for a couple of months in the freezer, but beware that they can become soggy and not taste quite the same after thawing.